This invention relates to a method of transmitting digital signals, and more particularly to a method of transmitting digital signals which is suitable for use in a recording and reproducing apparatus which effects after-recording by double recording.
Conventional recording and reproducing apparatuses of digital signals such as digital audio tape recorders employ a recording and reproducing method in which input data is interleaved in blocks of information, as disclosed, for example, in Japanese Patent Laid-Open No. 187039/1983. In apparatuses such as digital audio tape recorders, burst errors increase because the apparatuses effect high density recording. However, the recording and reproducing method described above converts the burst errors to random errors to improve the effect of error detection and correction codes, prevents preceding and subsequent data from simultaneously becoming errors even when correction of an error proves to be impossible, and makes approximation and interpolation by mean data of both the correct data. In the case of audio signals and video signals, the output signals after D/A conversion are not very offensive acoustically and visually.
In recording and reproducing apparatuses of digital signals such as floppy disks, however, an error of only one bit data results in a critical problem. Therefore, an omission of error detection and an erroneous detection must not by any means exist, and a method which changes the data such as mean value interpolation cannot be used.
Accordingly, it is customary in data recorders or the like to reduce the number of occurrence of errors by reducing the recording density in order to absolutely reduce the occurrence of error. For this reason, even if the recording and reproducing method by interleaving described above is employed, a digital signal processing circuit will become complicated in construction and large in scale so that not only the effective feature of the interleave cannot be used effectively but fresh problems occur.
The definite problems will now be described with reference to the case where an electronic camera (a still camera or a video floppy) system for recording still picture information on a floppy disk is used and digital data is recorded by this system in place of the picture, by way of example. (For the detail of the electronic camera, refer to Japanese Patent Laid-Open No. 84303/1984, for example. However, the problems to be described below are clarified as a result of studies made by the inventors of the present invention, and are not at all disclosed in the prior art reference described above.)
FIGS. 1A to 1C show recording formats for data recording of an electronic camera. FIG. 1A shows a frame construction. One frame consists of 128 blocks. Portion 21 corresponds to a head contact start position, and a burst signal or the like is recorded therein as a margin. An ID portion is an area in which ID codes for control other than input signals are added. FIG. 1B shows the construction of one block. Symbol Sync represents a sync signal, BA denotes a block address and sub-code, and Parity denotes a check code for detecting errors of the BA portion. This check code is, for example, a simple parity formed by effecting Mod 2-addition of the block address and the sub-code for each bit.
PCM data denotes an area which divides and records the input data into 32 samples (each sample consisting of 8 bits or 32 samples having 256 bits in all). C.sub.1 and C.sub.2 denote areas which record first and second codes for detecting and correcting errors of the PCM data. For example, a Reed-Solomon Code is generated and recorded.
FIG. 1C represents a magnetic sheet 22 called a "video floppy disk", and a method which divides the sheet 22 into four sectors 23.about.26 as shown in the drawing is employed.
FIG. 2 shows a memory map using the conventional interleave method. In the drawing, BLOCK corresponds to 1 block of data having the construction shown in FIG. 1, and S represents an area for storing the sync signal, the block address, the sub-code and check codes for detecting the errors of the block address and the sub-code. D denotes a memory area for storing the input PCM data, and C.sub.1 and C.sub.2 denote first and second error detection and correction codes for the PCM data.
In accordance with the conventional interleave method, each input data in time sequence is delayed and is stored at a position represented by an arrow B in the drawing. The second code C.sub.2 is generated from the data positioned at the arrow B and is stored at a position of an arrow Q. Furthermore, the first code C.sub.1 is generated from the PCM data and the code C.sub.2 at a position of an arrow A, and is stored on the same block of the arrow A as represented by an arrow P. Here, the sync signal till the code C.sub.1 of each block are read out in the sequence of the arrows A, P and in the sequence of the block number and are then recorded. Therefore, the input time sequence data and the code C.sub.2 are interleaved in delay blocks as represented by the arrows B, Q and the code C.sub.1, which is completed in one block data, is generated and recorded. In accordance with this interleave recording method, however, a problem develops when recording by double recording is effected.
FIGS. 3A-3D show the case where the problem described above develops. In the drawings, symbols a.sub.1, a.sub.2, a.sub.3, . . . , a.sub.128 and b.sub.1, b.sub.2, b.sub.3. . . , b.sub.128 represent the blocks and their block numbers in each frame signal. Here, FIG. 3A shows the relation of position when the signal is recorded under a normal state, and the signal is recorded at a position T interposed by pulses TAC.sub.1 that are generated by the rotation of the magnetic sheet. Let's consider the case where recording is deviated from a position T, at which the recording must be originally made, as shown in FIG. 3B when after-recording is made by double recording from above the recorded portion as shown in FIG. 3A. In this case, the signal after after-recording is as shown in FIG. 3C and the portion represented by E corresponds to the finish portion of the after-recording signals (FIG. 3B), and hence the old block data a.sub.125, a.sub.126 are assumed to be errors. In this case, a.sub.127, a.sub.128 are the old data, but do not prove to be the errors when check is made by use of the C.sub.1 code, which is completed in one block data and is generated and recorded.
In order to detect also the signal that is after-recorded at a deviated position as shown in FIG. 3B at the time of reproduction, it is necessary to take in and process all the data contained in the area R including margins both before and after the area T. However, the new data b.sub.1, b.sub.2, b.sub.3. . . , b.sub.128 by after-recording and the old data a.sub.125, a.sub.126, a.sub.127 a.sub.128 that are left unerased, that is to say, the data for the 132 blocks, co-exist in this area R. Which 128 blocks among the 132 blocks which are generated by after-recording can not be discriminated from the result of detection by means of the C.sub.1 code. In other words, there has been the problem that erroneous correction of errors is made by use of data of 128 blocks of combinations other than the combination b.sub.1, b.sub.2, b.sub.3. . . , b.sub.128 such as a combination of 128 blocks of b.sub.5, b.sub.6, b.sub.7, . . . , b.sub.128 with a.sub.125, a.sub.126, a.sub.127 and a.sub.128. Therefore, the conventional apparatuses need a circuit and an apparatus for recording non-signals for a predetermined period as an after-recording margin as represented by an oblique line portion in FIG. 3D after recording is completed.